This invention relates to a disc-shaped recording medium on which information signals are pre-recorded or which enables information signals to be recorded thereon, such as an optical disc or a magneto-optical disc. More particularly, it relates to a hub structure of the disc-shaped recording medium which permits the recording medium to be chucked on a rotational driving device for the recording medium. In the description to follow, such a disc-shaped recording medium is referred to simply as an optical disc.
The optical disc is a recording medium on which desired information signals, such as music signals or picture signals, are pre-recorded at a high recording density, or which permits such information signals to be recorded thereon at a high recording density. An optical disc of an extremely small diameter, such as 64 mm or less, has been put to practical use. When loaded on the recording/reproducing apparatus, the optical disc is run in high-speed rotation by being loaded in its center portion on a disc rotating device. While the optical disc is being run in high-speed rotation, it is irradiated with a laser beam outgoing from an optical pickup device so that information signals recorded on an information signal recording section formed on its major surface may be reproduced. If the optical disc is a recordable disc, an external magnetic field, modulated in accordance with information signals to be recorded, is applied by an external magnetic field generating device on the optical disc, while a laser light outgoing from an optical pickup device is radiated on the information signal recording section for recording desired information signals.
In order for the laser light to be accurately radiated on fine recording tracks of the information signal recording section of the optical disc rotated at an elevated velocity, the optical disc is reliably unified to a disc table of the disc rotating device, and is chucked in a state in which it has its center of rotation positioned highly accurately at an axial center of the disc table. The conventional chucking system for the optical disc is a so-called magnet chucking system which enables the recording/reproducing apparatus to be reduced in thickness and which exploits the force of magnetic attraction of a magnet positioning and chucking the optical disc with high accuracy.
That is, with the magnet chuck system, a magnet is arranged on a disc table, and a magnetic plate, constituted as a metal plate, is mounted in a center aperture of the optical disc. Thus, when loaded on the recording/reproducing apparatus, the optical disc has its magnetic plate attracted by the magnet so that the optical disc is unified with the disc table and run in high-speed rotation by the disc rotating device.
In FIGS. 9 to 11, there is shown a conventional optical disc 100 to which the above-described magnet chuck system is applied and which is made up of a disc substrate 101 and a hub structure 103. The disc substrate 101 is made up of a pair of disc-shaped transparent substrates 101A, 101B formed of glass or transparent synthetic resin, such as polycarbonate. One 101A of the transparent substrates has a circumferentially extending information signal recording section on its surface to be bonded to the other transparent substrate 101B. The transparent substrate 101B is bonded to the transparent substrate 101A for covering the information recording section with the aid of e.g., a hot melt, with the major surface of the transparent substrate 101B operating as an information signal readout surface.
The transparent substrates 101A, 101B are formed at the center regions with coaxial center openings 102A, 102B, respectively. When the transparent substrates 101A, 101B are joined together, the center apertures 102A, 102B together define a center aperture 102, in which is mounted a hub structure 103.
The hub structure 10S is made up of a disc-shaped magnetic plate 104 for magnet clamping, formed of a magnetic material, such as metal, an inner rim fitting member 106 unified to a center opening 105 formed in the magnetic plate 104 and an outer rim fitting member 107 unified to an outer rim of the magnetic plate 104. The magnetic plate 104 is formed as a disc slightly smaller in diameter than the center aperture 102 of the disc substrate 101.
The inner rim fitting member 108 is formed of a synthetic resin material having high resistance against abrasion and high lubricating properties, such as polyacetal resin or fluorine-containing polycarbonate resin, and is unified to the center opening 105 of the magnetic plate 104 by outsert molding. The inner rim fitting member 108 is in the form of a ring having a center opening 108 operating as a spindle receiving hole passed through by a spindle shaft of a rotating device of the recording/reproducing apparatus.
The outer rim fitting member 107 is formed of a synthetic resin material capable of transmitting UV rays and exhibiting superior adhesiveness with respect to the disc substrate 101, such as a transparent polycarbonate resin which is the same material as that used for the disc substrate 101. The outer rim fitting member 107 is formed as a ring-shaped member unified by outsert molding to the outer rim of the magnetic plate 104. In this case, the outer rim fitting member 107 is outsert-molded so that plural upstanding pieces formed on the outer rim of the magnetic plate 104 are molded in the outer rim fitting member 107 has its outer shape and size larger than the diameter of the center opening 102.
The above-described hub structure 103 is combined with the disc substrate 101 with the center aperture 102 of the disc substrate 101 precisely centered with respect to the spindle receiving opening 108 of the inner rim fitting member 106, as shown in FIG. 10. The portion of the major surface of the disc substrate 101 combined with the hub structure 103, which is located around the outer rim of the center aperture 102, adapted to be bonded to the major surface of the outer rim fitting member 107, is coated with an adhesive, such as UV curable adhesive. The UV curable adhesive is irradiated with UV rays and thereby cured for unifying the disc substrate 101 and the hub structure 103 together for completing the optical disc 100.
The optical disc 100 is rotatably housed within the inside of a disc cartridge casing made up of an upper half and a lower half which are in the form of shallow square-shaped saucers and which are abutted and bonded to each other. The disc cartridge casing has a recording/reproducing aperture which permits the information recording section of the disc substrate 101 to be exposed to the outside across its inner and outer rims and via which an optical pickup unit of the recording/reproducing apparatus is intruded to a position facing the disc. The recording/reproducing aperture is usually closed by a shutter member adapted for being moved along the major surface of the disc cartridge casing. The mid portion of the disc cartridge casing has a disc table intruding opening via which the magnetic plate 104 constituting the hub structure 103 of the optical disc 100 is exposed to the outside.
When the disc cartridge is loaded on a disc cartridge loading section of the recording/reproducing apparatus, the disc table of the disc rotating device is intruded via the disc table intruding opening into the inside of the disc cartridge so that a magnet provided on the disc table is magnetically attracted by the magnetic plate 104 to effect magnet chucking. When the shutter member is moved for opening the recording/reproducing aperture, the optical pickup device is intruded into the inside of the disc cartridge. The optical disc 100, thus magnet chucked, is rotated at an elevated velocity by the disc rotating device, so that information signals are recorded on or reproduced from the information recording section by the operation of the optical pickup device.
When the disc cartridge is to be ejected, the magnetic plate 104 of the hub structure 103 is disengaged from the magnet of the disc table so as to be taken out from the disc cartridge loading section of the recording/reproducing apparatus. The force of disengaging the magnetic plate 104 from the magnet operates in such direction that the disc substrate 101 and the outer rim fitting member 107 of the hub structure 103, which are bonded together by the adhesive, tend to be separated from each other.
Thus, by repeating the operation of disengaging the optical disc 100 from the disc table of the disc rotating device, the bonding portion of the magnetic plate 104 to the disc substrate 101 is subjected to a load of separating the optical disc from the disc table thus deteriorating the adhesive to cause the hub structure 103 to be detached from the disc substrate 101.
If, while the disc cartridge is loaded on the recording/reproducing apparatus, the hub structure is detached from the disc substrate 101, it remains attracted to the magnet of the disc table after the disc cartridge is taken out of the recording/reproducing apparatus. When a normal disc cartridge is loaded on the recording/reproducing apparatus, it is impossible to chuck the optical disc because the hub structure detached from the previous optical disc remains attracted to the disc table.